The invention is related to a multi-mode electro-mechanical variable speed transmission in a powertrain, and to a method of operating the transmission and the powertrain. It is applicable to a wide variety of vehicles and power equipment.
To reduce fuel consumption and emission, hybrid vehicles combine an electric power plant with a conventional internal combustion engine. The internal combustion engine operates in a certain range of speed and power. Inside this range, there usually exists a smaller regime where the engine achieves the best performance. On the other hand, however, driving conditions vary enormously, not only in wheel speed but also in driving torque at the drive wheels. A combination of a given speed and torque defines a power state. Selectively operating the internal combustion engine and matching its power state with that of the drive wheels are the primary functions for a hybrid transmission.
The development of hybrid technology provides new avenues for achieving improved operation and match of power state of the internal combustion engine with the drive wheels. Among various power-train architectures, a well-known design is the electro-mechanical continuous variable transmission, known as Toyota hybrid system, or THS. THS allows for electric propulsion at low power and slow speed operation and turns on the engine in hybrid operation when vehicle speed and or power demands exceed certain thresholds. In the hybrid operation, THS splits the input power into two paths of different nature. Part of the input power passes through a so-called mechanical power path which is comprised of gears and shafts; the rest of the input power passes through an electric power path which contains electric machines and inverters. The device used to split the power is a simple planetary gear system. THS offers only one power splitting mode and provides a single output to input speed ratio node point SR. When the transmission operates at a speed ratio higher than the speed ratio node point, internal power circulation occurs. One of the power paths sees more power than that is transmitted through the transmission, which reduces power transmission efficiency. This, to a large extent, constrains the effective operating speed ratio of the transmission. For high power vehicle applications, the power ratings for the electric machines have to be increased significantly. In the electric drive mode, only one of the electric machines provides motive power. This makes THS not suitable for all electric drive in power demanding applications. An example of such an application is all electric range (AER) plug-in hybrid configurations where the vehicles operate in pure electric mode until the battery charge is depleted below a predetermined threshold.